The present invention relates to tough TiCN and/or TiC cermets and more particularly to cermets having improved toughness due to their microstructural features.
In general, cermets comprise as ceramic components oxides, carbides, borides, nitrides, etc. of elements of the IVa, Va or VIa group of the Periodic Table and as binder components metals such as cobalt, nickel, molybdenum, etc. Among others, TiCN cermets, TiC cermets or TiCN-TiC cermets are useful because of their high wear resistance. Such cermets are generally made from TiCN and/or TiC, WC and binder metals.
Japanese Patent Publication No. 56-51201 discloses cemented carbonitride alloys consisting of carbonitrides of titanium, tungsten and other optional elements and binders of the iron-group groups. These cemented carbonitride alloys which may also be called simply cermets have a grain microstructure consisting of two phases: a carbonitride solid solution phase rich in titanium and nitrogen, and another hardening phase surrounding the above solid solution phase and rich in metal components of the VI group but scarce of nitrogen. Thus, the hardening phase forms a boundary phase which is in contact with a metal binder phase. Although the carbonitride solid solution phase has a poor wettability to the iron-group metals, the boundary phase surrounding the carbonitride solid solution phase is highly wettable with the iron-group metals. Therefore, the boundary phase serves to bond the carbonitride core phase and the metal binder phase.
This conventional cermet is prepared by first preparing a solid solution from titanium carbide, titanium nitride and tungsten carbide at high temperatures and high pressure, pulverizing it into fine powder after cooling, uniformly mixing carbonitride fine powder with binder metals, pressing the resulting mixture to form a green body, and sintering the green body at high temperatures. Why the two-phase grain microstructure appears is, according to the inventor of Japanese Patent Publication No. 56-51201, that a spinodal reaction takes place in carbonitride powder while simultaneously a diffusion reaction occurs in the liquified binder phase in the sintering process, resulting in the carbonitride phase surrounded by a carbide phase containing little nitrogen. The carbide phase forms a low-stress boundary phase in contact with the metal binder phase.
In this conventional cermet, the carbonitride core phase is hard and so has a high wear resistance, but it is brittle and so vulnerable to cracking. On the other hand, the boundary phase surrounding the carbonitride core phase has somewhat higher toughness but it is poor in wear resistance. Because the toughness of the surrounding boundary phase is not enough to stop the propagation of cracks from one core phase to another, cracks, once created, tend to grow rather straight within the entire body of the cermet, passing through the carbonitride core phase one after another. Microscopically speaking, a crack created in a carbonitride boundary phase cannot be stopped to propagate by the surrounding boundary phase because of high brittleness of the core phase and insufficiency in the toughness of the boundary phase, resulting in cracking in an adjacent core phase. This phenomenon takes place throughout the cermet, meaning that this cermet is not sufficiently tough despite its high wear resistance.